1. Field of the Invention
This invention relates generally to tractors for moving equipment within passages.
2. Description of the Related Art
The art of moving equipment through vertical, inclined, and horizontal passages plays an important role in many industries, such as the petroleum, mining, and communications industries. In the petroleum industry, for example, it is often required to move drilling, intervention, well completion, and other forms of equipment within boreholes drilled into the earth.
One method for moving equipment within a borehole is to use rotary drilling equipment. In traditional rotary drilling, vertical and inclined boreholes are commonly drilled by the attachment of a rotary drill bit and/or other equipment (collectively, the xe2x80x9cBottom Hole Assemblyxe2x80x9d or BHA) to the end of a rigid drill string. The drill string is typically constructed of a series of connected links of drill pipe that extends between ground surface equipment and the BHA. A passage is drilled as the drill string and drill bit are together lowered into the earth. A drilling fluid, such as drilling mud, is pumped from the ground surface equipment through an interior flow channel of the drill string to the drill bit. The drilling fluid is used to cool and lubricate the bit, and only recently for drilling to remove debris and rock chips from the borehole, which are created by the drilling process. The drilling fluid returns to the surface, carrying the cuttings and debris, through the annular space between the outer surface of the drill pipe and the inner surface of the borehole. As the drill string is lowered or raised within the borehole, it is necessary to continually add or remove links of drill pipe at the surface, at significant time and cost.
Another method of moving equipment within a borehole involves the use of a downhole tool, such as a tractor, capable of gripping onto the borehole and thrusting both itself and other equipment through it. Such tools can be attached to rigid drill strings, but can also be used in conjunction with coiled tubing equipment. Coiled tubing equipment includes a non-rigid, compliant tube, referred to herein as xe2x80x9ccoiled tubing,xe2x80x9d through which operating fluid is delivered to the tool. The operating fluid provides hydraulic power to propel the tool and the equipment and, in drilling applications, to lubricate the drill bit. The operating fluid also can provide the power for gripping the borehole. In comparison to rotary equipment, the use of coiled tubing equipment in conjunction with a tractor should be generally less expensive, easier to use, less time consuming to employ, and should provide more control of speed and downhole loads. Also, a tractor, which thrusts itself within the passage and pushes and pulls adjoining equipment and coiled tubing, should move more easily through inclined or horizontal boreholes. In addition, due to its greater compliance and flexibility, the coiled tubing permits the tractor to perform much sharper turns in the passage than rotary equipment.
A tractor can be utilized for drilling boreholes as well as many other applications, such as well completion and production work for producing oil from an oil well, pipeline installation and maintenance, laying and movement of communication lines, well logging activities, washing and acidizing of sands and solids, retrieval of tools and debris, and the like.
One type of tractor comprises an elongated body securable to the lower end of a drill string. The body can comprise one or more connected shafts in addition to a control assembly housing or valve system. This tractor includes at least one anchor or gripper assembly adapted to grip the inner surface of the passage. When the gripper assembly is actuated, hydraulic power from operating fluid supplied to the tractor via the drill string can be used to force the body axially through the passage. The gripper assembly is longitudinally movably engaged with the tractor body, so that the body and drill string can move axially through the passage while the gripper assembly grips the passage surface. A gripper assembly can transmit axial and even torsional loads from the tractor body to the borehole wall. Several highly effective designs for a fluid-actuated gripper assembly are disclosed in U.S. patent application Ser. No. 09/777,421. In one design, the gripper assembly includes a plurality of flexible toes that bend radially outward to grip onto the passage surface by the interaction of ramps and rollers.
Some tractors have two or more sets of gripper assemblies, which permits the tractor to move continuously within the passage. Forward longitudinal motion (unless otherwise indicated, the terms xe2x80x9clongitudinalxe2x80x9d and xe2x80x9caxialxe2x80x9d are herein used interchangeably and refer to the longitudinal axis of the tractor body) is achieved by powering the tractor body forward with respect to an actuated first gripper assembly (a xe2x80x9cpower strokexe2x80x9d with respect to the first gripper assembly), and simultaneously moving a retracted second gripper assembly forward with respect to the tractor body (a xe2x80x9creset strokexe2x80x9d of the second gripper assembly). At the completion of the power stroke with respect to the first gripper assembly, the second gripper assembly is actuated and the first gripper assembly is retracted. Then, the tractor body is powered forward while the second gripper assembly is actuated (a power stroke with respect to the second gripper assembly), and the retracted first gripper assembly executes a reset stroke. At the completion of these respective strokes, the first gripper assembly is actuated and the second gripper assembly is retracted. The cycle is then repeated. Thus, each gripper assembly operates in a cycle of actuation, power stroke, retraction, and reset stroke, resulting in longitudinal motion of the tractor. A number of highly effective tractor designs utilizing this configuration are disclosed in U.S. Pat. No. 6,003,606 to Moore et al., which discloses several embodiments of a tractor known as the xe2x80x9cPuller-Thruster Downhole Tool;xe2x80x9d U.S. Pat. No. 6,241,031 to Beaufort et al.; and allowed U.S. patent application Ser. No. 09/453,996, which discloses an xe2x80x9cElectrically Sequenced Tractorxe2x80x9d (xe2x80x9cESTxe2x80x9d).
The power required for actuating the gripper assemblies, longitudinally thrusting the tractor body during power strokes, and longitudinally resetting the gripper assemblies during reset strokes may be provided by pressurized operating fluid delivered to the tractor via the drill stringxe2x80x94either a rotary drill string or coiled tubing. For example, the aforementioned Puller-Thruster Downhole Assembly includes inflatable engagement bladders and uses hydraulic power from the operating fluid to inflate and radially expand the bladders so that they grip the passage surface. Hydraulic power is also used to move forward cylindrical pistons residing within sets of propulsion cylinders slidably engaged with the tractor body. Each set of cylinders is secured with respect to a bladder, so that the cylinders and bladder move together longitudinally. Each piston is longitudinally fixed with respect to the tractor body. When a bladder is inflated to grip onto the passage wall, operating fluid is directed to the proximal side of the pistons in the set of cylinders secured to the inflated bladder, to power the pistons forward with respect to the borehole. The forward hydraulic thrust on the pistons results in forward thrust on the entire tractor body. Further, hydraulic power is also used to reset each set of cylinders when their associated bladder is deflated, by directing drilling fluid to the distal side of the pistons within the cylinders.
A tractor can include a valve system for, among other functions, controlling and sequencing the distribution of operating fluid to the tractor""s gripper assemblies, thrust chambers, and reset chambers. Some tractors, including several embodiments of the Puller-Thruster Downhole Tool, are all-hydraulic. In other words, they utilize pressure-responsive valves and no electrically controlled valves. One type of pressure-responsive valve shuttles between its various positions based upon the pressure of the operating fluid in various locations of the tractor. In one configuration, a spool valve is exposed on both ends to different fluid chambers or passages. The valve position depends on the relative pressures of the fluid chambers. Fluid having a higher pressure in a first chamber exerts a greater pressure force on the valve than fluid having a lower pressure in a second chamber, forcing the valve to one extreme position. The valve moves to another extreme position when the pressure in the second chamber is greater than the pressure in the first chamber. Another type of pressure-responsive valve is a spring-biased spool valve having at least one end exposed to fluid. The fluid pressure force is directed opposite to the spring force, so that the valve is opened or closed only when the fluid pressure exceeds a threshold value.
Other tractors utilize valves controlled by electrical signals sent from a control system at the ground surface or even on the tractor itself. For example, the aforementioned EST includes both electrically controlled valves and pressure-responsive valves. The electrically controlled valves are controlled by electrical control signals sent from a controller housed within the tractor body. The EST is preferred over all-hydraulic tractors for drilling operations, because electrical control of the valves permits very precise control over important drilling parameters, such as speed, position, and thrust. In contrast, all-hydraulic tractors, including several embodiments of the Puller-Thruster Downhole Tool, are preferred for so-called xe2x80x9cinterventionxe2x80x9d operations. As used herein, xe2x80x9cinterventionxe2x80x9d refers to re-entry into a previously drilled well for the purpose of improving well production, to thereby improve fuel production rates. As wells age, the rate at which fuel can be extracted therefrom diminishes for several reasons. This necessitates the xe2x80x9cinterventionxe2x80x9d of many different types of tools. Hydraulic tractors, as opposed to electrically controlled tractors, are preferred for intervention operations because intervention, as opposed to drilling, does not require precise control of speed or position. The absence of electrically controlled valves makes hydraulic tractors generally less expensive to deploy and operate.
Tractors in combination with coiled tubing equipment are particularly useful for intervention operations because, in many cases, the wells were originally drilled with rotary drilling equipment capable of drilling very deep holes. It is more expensive to bring back the rotary equipment than it is to bring in a coiled tubing unit. However, the coiled tubing unit may not be capable of reaching extended distances within the borehole without the aid of a tractor.
In one known design, exemplified by FIG. 3 of U.S. Pat. No. 6,003,606 (which discloses the Puller-Thruster Downhole Tool), a tractor includes a spool valve whose spool has two main positions. In one main position, the valve directs pressurized fluid to a first gripper and to propulsion chambers of a first set of propulsion cylinders. In this position of the spool, the pressure is permitted to decrease in a second gripper and in reset chambers of a second set of propulsion cylinders. In the other main position, the valve does the oppositexe2x80x94it directs pressurized fluid to the second gripper and propulsion chambers of the second set of cylinders, and permits pressure to decrease in the first gripper and in propulsion chambers of the first set of cylinders. The spool of the valve is piloted by fluid pressure on both ends of the spool. A pair of cycle valves selectively administers high pressure to the ends of the spool. Each cycle valve is in turn piloted by the pressure in the fluid passages to the cylinders and grippers.
The Puller-Thruster all-hydraulic tractor design has proven to be a major advance in the art of tractors for moving equipment within boreholes. However, it operates most effectively within a limited zone of parameters, including the pressure, weight, and density of the operating fluid, the geometry of the tractor components, and the total weight of the equipment that the tractor must pull and/or push. Thus, it is desirable to provide an improved design for a tractor, which will work within a much larger zone of such parameters.
Another prior design consists of a wellbore tractor having wheels that roll along the surface of the well casing. This design is problematic because the wheels do not have the ability to provide significant gripping force to move heavier downhole equipment. Also, the wheels can lose traction in certain conditions, such as in regions including sand.
A typical process of extracting hydrocarbons from the earth involves drilling an underground borehole and then inserting a generally tubular casing in the borehole. In order to access oil reserves from a given underground region through which the well passes, the casing must be opened within that region. In one method, perforation guns are brought to the desired location within the well and then utilized to cut openings through the casing wall and/or the earth formation. Oil is then extracted through the openings in the casing up through the well to the surface for collection. Perforation guns can also be used to penetrate the formation in an xe2x80x9copen holexe2x80x9d to access desired oil reserves. An open hole is a borehole without a casing. Perforation guns can be ignited by different means, such as by pressurized operating fluid or electricity provided through electrical lines (xe2x80x9ce-linesxe2x80x9d). However, the practice of igniting the perforation guns with e-lines poses the risk of a spark leading to explosion and potential loss of life. Thus, it is desirable to fully hydraulic tractors, without e-lines, for operations that involve the use of perforation guns.
Perforation guns are commonly used in conjunction with rotary drilling equipment, due to the large weight of the guns. Long strips of perforation guns can weigh up to 20000 pounds or more. The rotary drilling equipment, consisting of the rigid drill string formed from connected links of drill pipe, has been used because of its ability to absorb the weight in tension. However, the use of rotary equipment is very expensive and time-consuming, due in part to the necessity of assembling and disassembling the portions of drill pipe.
In the prior art, shafts designed for downhole tools used in drilling and intervention applications have been formed from more flexible materials, such as copper beryllium (CuBe). This is because in drilling it is not uncommon to experience sharp turns, and the tool is preferably capable of turning at sharp angles. Also, shafts have been formed with relatively large internal passages for the flow of operating fluid to the valves and other equipment of the BHA. This is because in drilling the operating fluid is typically drilling mud, which often contains larger solids and necessitates a larger flow passage. The drilling mud is preferred because it provides better lubrication to the drill bit and more effectively carries the drill cuttings up through the annulus back to the ground surface.
The shaft of a downhole tool typically must include multiple internal passages (e.g., for fluid to the gripper assemblies, propulsion chambers, and the other downhole equipment) that extend along the shaft length. In the past, such passages have been formed by gun-drilling, which is well known. Unfortunately, it is typically not possible to gun-drill the entire length of the shaft (in most applications, the length of a shaft for a downhole tool can be anywhere in the range of 50 to 168 inches). The distance that a passage can be gun-drilled is limited by (1) the inherent length limitations of known gun-drilling tools, and (2) the limitations imposed by the geometry and material characteristics of the shaft. In the past, it has been necessary to limit the length of gun-drilled passages in shafts of downhole tools to a relatively great degree. This is because the larger internal passage required for drilling mud leaves less room for other fluid passages. This shortage of available xe2x80x9creal estatexe2x80x9d in the shaft requires higher precision gun-drilling and increases the risk of inadvertent damage to other passages caused by the gun-drilling process. These problems are exacerbated by the fact that the more flexible materials used for the shaft (e.g., CuBe) are softer, more difficult to drill through, and more prone to damage.
The limitations on the length that passages can be gun-drilled have necessitated forming the shafts from a plurality of shaft portions of reduced length. The fluid passages are gun-drilled in each shaft portion, and then the shaft portions are attached to each other. Due in large part to the use of CuBe, shaft portions have been attached together by electron beam welding. Electron beam welding is favored because it maintains the structural integrity of the material and of the fluid passages contained therein. Unfortunately, electron beam welding is a very expensive process. Most conventional welding processes have not been used because they do not facilitate the welding together of thick objects (i.e., the weld does not fuse completely through the objects). In shaft manufacturing for downhole tools, it is necessary to soundly fuse together all of the mating surfaces in order to maintain zero leakage between the various internal fluid passages and to provide structural integrity.
The present invention seeks to overcome the aforementioned limitations of the prior art by providing a hydraulically powered and substantially or completely hydraulically controlled tractor to be used preferably with coiled tubing equipment. This invention represents a major advancement in the art of tractors, and particular in the art of well intervention tools. Compared to the prior art, the preferred embodiments of the tractor of the invention operate very effectively within a much larger zone of parameters, such as the pressure, weight, and density of the operating fluid, the geometry of the tractor components, and the total weight of the equipment that the tractor must pull and/or push.
As explained below, the tractor preferably includes a two-position propulsion control valve that directs fluid to and from the tractor""s propulsion cylinders. In order for the propulsion control valve spool to shift, two cycle valves are provided for sensing the completion of the strokes of the propulsion cylinders. The cycle valves shift in order to begin a sequence of events that results in a fluid pressure force causing the propulsion control valve spool to shift, so that the propulsion cylinders can switch between their power and reset strokes. However, rather than administering high pressure fluid directly to the propulsion control valve spool, the cycle valves shift to send a pressure force to an additional two-position valve. The additional valve controls the flow of pressurized fluid to control the position of the propulsion control valve spool. Thus, the additional valve isolates the propulsion control valve from direct interaction with the cycle valves. Advantageously, the shift action of the additional valve creates a longer time lag between the shift action of either cycle valve and the shift action of the propulsion control valve spool. Due to the time lag, the propulsion cylinders are more likely to complete their strokes before the propulsion control valve shifts. In addition, better shifting can be effected by spring-assisted detents on the propulsion control valve spool. In the illustrated embodiments of the invention, the additional valve comprises a gripper control valve that controls the distribution of fluid to and from the gripper assemblies.
The preferred embodiments include an inlet control valve having a feature that allows the valve to be hydraulically restrained in a closed position, so that the tractor is assured of being non-operational and in a non-gripping state. This permits the operation of downhole equipment adjoined to the tractor or other portions of the bottom hole assembly, such as perforation guns, substantially without the risk of inadvertent movement of the tractor. It also assures that the gripper assemblies are retracted from the borehole surface during the operation of other downhole equipment, thus reducing the risk of damage to the gripper assemblies.
In addition, the invention provides a new method of manufacturing the shafts that form the body of the tractor, which is much less expensive than prior art shaft manufacturing methods. According to this method, shaft portions are silver brazed together to form the shafts. Silver brazing is less expensive than prior art welding methods, such as electron beam welding. Also, the preferred material characteristics and internal fluid passage configuration permits longer gun-drilled holes. Advantageously, fewer shaft portions are necessary.
In one aspect, the present invention provides a tractor assembly comprising a tractor for moving within a borehole. The tractor comprises an elongated body, first and second gripper assemblies, first and second elongated propulsion cylinders, and a valve system. The body has first and second pistons longitudinally fixed with respect to the body. Each piston has aft and forward surfaces configured to receive longitudinal thrust forces from fluid from a pressurized source. The body has a flow passage.
Each gripper assembly is longitudinally movably engaged with the body. Each gripper assembly has an actuated position in which the gripper assembly limits relative movement between the gripper assembly and an inner surface of the borehole, and a retracted position in which the gripper assembly permits substantially free relative movement between the gripper assembly and said inner surface. Each gripper assembly is configured to be actuated by fluid.
The first propulsion cylinder is longitudinally slidably engaged with respect to the body and has an elongated internal propulsion chamber enclosing the first piston. The first piston is slidable within and fluidly divides the internal propulsion chamber of the first cylinder into an aft chamber and a forward chamber. Similarly, the second propulsion cylinder is longitudinally slidably engaged with respect to the body and has an elongated internal propulsion chamber enclosing the second piston. The second piston is slidable within and fluidly divides the internal propulsion chamber of the second cylinder into an aft chamber and a forward chamber.
The valve system comprises a propulsion control valve and a gripper control valve. The propulsion control valve has a first position in which it provides a flow path for the flow of fluid to the aft chamber of the first cylinder. The propulsion control valve also has a second position in which it provides a flow path for the flow of fluid to the aft chamber of the second cylinder. The gripper control valve has a first position in which it provides a flow path for the flow of fluid to the first gripper assembly. The gripper control valve also has a second position in which it provides a flow path for fluid to the second gripper assembly. When the gripper control valve is in its first position and the propulsion control valve is in its first position, the gripper control valve must move from its first position to its second position before the propulsion control valve can move from its first position to its second position.
In another aspect, the present invention provides a method of moving the tractor assembly (described immediately above) within a borehole. The method comprises providing pressurized fluid from a source, directing the pressurized fluid toward the gripper control valve, directing the pressurized fluid toward the propulsion valve, and, when the gripper control valve and propulsion control valves are in their first positions, preventing the propulsion control valve from moving from its first position to its second position until the gripper control valve moves from its first position to its second position.
In another aspect, the invention provides a tractor assembly, comprising a tractor for moving within a borehole. The tractor comprises an elongated body, first and second gripper assemblies, first and second elongated propulsion cylinders, and a valve system. The elongated body has first and second pistons longitudinally fixed with respect to the body. Each of the pistons has aft and forward surfaces configured to receive longitudinal thrust forces from fluid from a pressurized source. The body also has a flow passage. Each of the first and second gripper assemblies is longitudinally movably engaged with the body, and has actuated and retracted positions as described above. The first and second propulsion cylinders are configured as described above.
The valve system comprises a propulsion valve and a control valve. The propulsion valve has a first position in which it provides a flow path for the flow of fluid to the aft chamber of the first cylinder, and a second position in which it provides a flow path for the flow of fluid to the aft chamber of the second cylinder. The control valve has a first position in which it provides a flow path for the flow of fluid to urge the propulsion valve toward the first position of the propulsion valve. The control valve has a second position in which it provides a flow path for the flow of fluid to urge the propulsion valve toward the second position of the propulsion valve. When the control valve and the propulsion valve are in their first positions, the control valve must move from its first position to its second position before the propulsion valve can move from its first position to its second position.
In another aspect, the invention provides a method of moving the tractor assembly (described immediately above) within a borehole. The method comprises providing pressurized fluid from a source, directing the pressurized fluid toward the gripper control valve, directing the pressurized fluid toward the propulsion valve, and, when the control valve and the propulsion valve are in their first positions, preventing the propulsion valve from moving from its first position to its second position before the control valve moves from its first position to its second position.
In another aspect, the invention provides a tractor assembly, comprising a tractor for moving within a borehole. The tractor is configured to be powered by operating fluid received from a conduit extending from the tractor through the borehole to a source of the operating fluid. The tractor comprises an elongated body, a gripper assembly, a valve system housed within the body, a pressure reduction valve, and first and second gripper fluid passages. The elongated body has a thrust-receiving portion longitudinally fixed with respect to the body. The body also has an internal passage configured to receive the operating fluid from the conduit. The gripper assembly is longitudinally movably engaged with the body and has actuated and retracted positions as described above. The valve system is configured to receive operating fluid from the internal passage of the body and to selectively control the flow of operating fluid to at least one of the gripper assembly and the thrust-receiving portion. The first gripper fluid passage extends from the valve system to the pressure reduction valve, while the second gripper fluid passage extends from the pressure reduction valve to the gripper assembly. The pressure reduction valve is configured to provide a flow path for operating fluid to flow from the first gripper fluid passage to the second gripper fluid passage when the pressure within the first gripper fluid passage is below a threshold. The pressure reduction valve is also configured to prevent fluid from flowing from the first gripper fluid passage to the second gripper fluid passage when the pressure within the first gripper fluid passage is above the threshold.
In another aspect, the invention provides a method of moving a tractor assembly within a borehole. The tractor assembly includes a tractor having an elongated body, a gripper assembly longitudinally movably engaged with the body, a valve system housed within the body, and first and second gripper fluid passages. The body has a thrust-receiving portion longitudinally fixed with respect to the body. The body also has an internal passage configured to receive the operating fluid from the conduit. The gripper assembly has actuated and retracted positions as described above, and is configured to be actuated by receiving operating fluid from the internal passage of the body. The valve system is configured to receive operating fluid from the internal passage of the body and to selectively control the flow of operating fluid to at least one of the gripper assembly and the thrust-receiving portion. The first gripper fluid passage extends from the valve system, and the second gripper fluid passage extends to the gripper assembly. According to the method of this aspect of the invention, pressurized fluid is provided from a source. The pressurized fluid is permitted to flow from the first gripper fluid passage to the second gripper fluid passage when the pressure within the first gripper fluid passage is below a threshold. Fluid is prevented from flowing from the first gripper fluid passage to the second gripper fluid passage when the pressure within the first gripper fluid passage is above the threshold.
In another aspect, the invention provides a tractor assembly, comprising a tractor for moving within a borehole. The tractor is configured to be powered by pressurized operating fluid received from a conduit extending from the tractor through the borehole to a source of the operating fluid. The tractor comprises an elongated body, a gripper assembly longitudinally movably engaged with the body, and a valve system housed within the body. The body has a thrust-receiving portion longitudinally fixed with respect to the body, and an internal passage configured to receive the operating fluid from the conduit. The gripper assembly has actuated and retracted positions as described above.
The valve system is configured to receive fluid from the internal passage of the body and to selectively control the flow of operating fluid to at least one of the gripper assembly and the thrust-receiving portion. The valve system includes an entry control valve controlling the flow of operating fluid from the internal passage of the body into the valve system. The entry control valve comprises a valve passage and a body movably received therein. The valve passage has at least two secondary passages and is configured to conduct the operating fluid between the secondary passages. The entry control valve has first and third position ranges in which it provides a flow path for operating fluid within the valve system to flow through the entry control valve to the exterior of the tractor, and in which the valve body prevents the flow of operating fluid from the internal passage of the tractor body into the valve system. The entry control valve also has a second position range in which it provides a flow path for operating fluid from the internal passage of the tractor body to flow into the valve system, and in which the valve body prevents the flow of operating fluid within the valve system to the exterior of the tractor. The entry control valve is in its first position range when the fluid pressure in the internal passage of the tractor body is below a lower shut-off threshold. The entry control valve is in the second position range when the fluid pressure in the internal passage is above the lower shut-off threshold and below an upper shut-off threshold. The entry control valve is in the third position range when the fluid pressure in the internal passage is above the upper shut-off threshold.
In another aspect, the invention provides a method of moving a tractor assembly within a borehole, the tractor assembly including a tractor having an elongated body and gripper assembly configured as in the previously described aspect of the invention. The tractor also comprises a valve system housed within the body, the valve system including an entry control valve. According to the method, fluid is received from the internal passage of the body, and the flow of operating fluid from the internal passage of the body into the valve system is controlled with the entry control valve. The flow of operating fluid from the internal passage of the body into the valve system is prevented with the entry control valve when the fluid pressure in the internal passage of the body is below a lower shut-off threshold and when the fluid pressure in the internal passage is above an upper shut-off threshold. The flow of operating fluid from the internal passage of the body into the valve system is permitted when the fluid pressure in the internal passage is above the lower shut-off threshold and below the upper shut-off threshold.
In another aspect, the present invention provides a tractor assembly, comprising a tractor for moving within a borehole. The tractor is configured to be powered by pressurized operating fluid received from a conduit extending from the tractor through the borehole to a source of the operating fluid. The tractor comprises an elongated body, a gripper assembly longitudinally movably engaged with the body, and a valve system. The elongated body has a thrust-receiving portion longitudinally fixed with respect to the body. The body also has an internal passage configured to receive the operating fluid from the conduit. The gripper assembly has actuated and retracted positions as described above.
The valve system of the tractor is configured to receive fluid from the internal passage of the body and to selectively control the flow of operating fluid to at least one of the gripper assembly and the thrust-receiving portion. The valve system includes an entry control valve controlling the flow of operating fluid from the internal passage of the body into the valve system. The entry control valve comprises a housing defining a valve passage, a body movably received within the passage, and at least one spring. The housing has at least two side passages, the valve passage being configured to conduct the operating fluid between the side passages. The valve body has a first surface configured to be exposed to operating fluid from the internal passage of the tractor body, the first surface being configured to receive a longitudinal pressure force in a first direction. The valve body has first and third position ranges in which the body provides a flow path for operating fluid within the valve system to flow through the entry control valve to the exterior of the tractor, and in which the valve body prevents the flow of operating fluid from the internal passage of the body into the valve system. The valve body has a second position range between the first and third position ranges in which the valve body provides a flow path for operating fluid from the internal passage of the tractor body to flow into the valve system, and in which the valve body prevents the flow of operating fluid within the valve system to the exterior of the tractor.
The at least one spring biases the valve body in a direction opposite to that of the pressure force received by the first surface of the valve body, such that the magnitude of the fluid pressure in the internal passage determines the deflection of the at least one spring and thus the position of the valve body. The at least one spring is configured so that the valve body occupies a position within the first position range when the fluid pressure in the internal passage of the tractor body is below a lower shut-off threshold, so that the valve body occupies a position within the second position range when the fluid pressure in the internal passage is above the lower shut-off threshold and below an upper shut-off threshold, and so that the valve body occupies a position within the third position range when the fluid pressure in the internal passage is above the upper shut-off threshold.
In another aspect, the invention provides a tractor assembly, comprising a tractor for moving within a borehole while connected to an injector by a drill string. The tractor comprises an elongated body, first and second gripper assemblies, elongated first and second propulsion cylinders, and a valve system. The body has first and second pistons longitudinally fixed with respect to the body. Each of the pistons has aft and forward surfaces configured to receive longitudinal thrust forces from fluid from a pressurized source. The body also has a flow passage. The first gripper assembly is longitudinally movably engaged with the body and has actuated and retracted positions as described above. Similarly, the second gripper assembly is longitudinally movably engaged with the body and has actuated and retracted positions as described above. The first propulsion cylinder is longitudinally slidably engaged with respect to the body. The first cylinder has an elongated internal propulsion chamber enclosing the first piston. The first piston is slidable within and fluidly divides the internal propulsion chamber of the first cylinder into an aft chamber and a forward chamber. Similarly, the second propulsion cylinder is longitudinally slidably engaged with respect to the body. The second cylinder has an elongated internal propulsion chamber enclosing the second piston. The second piston is slidable within and fluidly divides the internal propulsion chamber of the second cylinder into an aft chamber and a forward chamber.
The valve system of the tractor comprises a propulsion control valve and a gripper control valve. The propulsion control valve has a first position in which it provides a flow path for the flow of fluid to the aft chamber of the first cylinder, and a second position in which it provides a flow path for the flow of fluid to the aft chamber of the second cylinder. The gripper control valve has a first position in which it provides a flow path for the flow of fluid to the first gripper assembly, and a second position in which it provides a flow path for fluid to the second gripper assembly. The speed of movement of the tractor is controlled by the pressure and flow rate of the operating fluid and the tension exerted on the tractor by the drill string.
In another aspect, the invention provides a tractor assembly, comprising a tractor for moving within a borehole. The tractor comprises an elongated body, a first gripper assembly longitudinally movably engaged with the body, an elongated first propulsion cylinder longitudinally slidably engaged with respect to the body, and a valve system. The body has first and second pistons longitudinally fixed with respect to the body. Each of the pistons has aft and forward surfaces configured to receive longitudinal thrust forces from fluid from a pressurized source. The body also has a flow passage. The first gripper assembly has actuated and retracted positions as described above. The first propulsion cylinder has an elongated internal propulsion chamber enclosing the first piston. The first piston is slidable within and fluidly divides the internal propulsion chamber of the first cylinder into an aft chamber and a forward chamber.
The valve system comprises a propulsion valve and a control valve. The propulsion valve has a first position in which it provides a flow path for the flow of fluid to the aft chamber of the first cylinder, and a second position in which it does not provide a flow path for the flow of fluid to the aft chamber of the first cylinder. The control valve has a first position in which it provides a flow path for the flow of fluid to urge the propulsion valve toward the first position, and a second position in which it provides a flow path for the flow of fluid to urge the propulsion valve toward the second position. When the control valve and the propulsion valve are in their first positions, the control valve must move from its first position to its second position before the propulsion valve can move from its first position to its second position.
For purposes of summarizing the invention and the advantages achieved over the prior art, certain objects and advantages of the invention have been described above and as further described below. Of course, it is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.
All of these embodiments are intended to be within the scope of the invention herein disclosed. These and other embodiments of the present invention will become readily apparent to those skilled in the art from the following detailed description of the preferred embodiments having reference to the attached figures, the invention not being limited to any particular preferred embodiment(s) disclosed.